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H.-Y. Wen, C.-M. Chao, M.-Y. Chang,*C.-W. Hsieh, S.-C. Hsieh

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A Spirobifluorene Derivative asa Single-Emitting Component fora Highly Efficient White OrganicElectroluminescent Device

All aglow: High-brightness, broad-spec-trum, white-light organic light-emittingdiodes, incorporating a single emissionlayer of the blue-light-emitting material2,2’,7,7’-tetrakis(pyren-1-yl)-9,9’-spirobi-fluorene (TPSBF), which comprises fourpyrene units linked to a spirobifluorenecore, are described (see figure).

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DOI: 10.1002/cplu.201300088

A Spirobifluorene Derivative as a Single-EmittingComponent for a Highly Efficient White OrganicElectroluminescent DeviceHsin-Yi Wen,[a] Chun-Ming Chao,[a] Mei-Ying Chang,*[a] Chiung-Wen Hsieh,[b] andShu-Chen Hsieh[b]

Introduction

Organic light-emitting devices (OLEDs), exhibiting high bright-ness, full-color emissions, and rapid responses, can be used tofabricate large-area displays and flexible thin-film devices.White organic light-emitting devices (WOLEDs) are particularlyattractive because of their applications as inexpensive back-lights and displays and for their potential use in solid-statelighting.[1–4]

WOLEDs, which require the emissions of three primarycolors (red, green, and blue), can be prepared by means of var-ious strategies, most commonly through the fabrication of1) multiple emission-layer devices through consecutive evapo-ration (with each layer emitting a primary color) ;[5] 2) single-layer polymer blend devices[6–9] (in which all of the emittingcomponents are mixed in one layer) ; and 3) single-layer struc-tures, in which dyes emitting different colors are doped ina host.[10–12]

The emission colors obtained from multi- and single-layerpolymer devices are typically sensitive to the device structureparameters (e.g. , active layer thickness, doping concentration),potentially increasing the unit manufacturing cost. Such sys-

tems have not produced satisfactory white-light emissions interms of either color or efficiency.[13–17]

Emissions of white light can also be achieved by usinga single emitting component. Early in 1997, Yang and Pei re-ported a single-emitting-component WOLED that featureda polyfluorene derivative, poly[9,9-bis(3,6-dioxahepty)fluorene-2,7-diyl] (BDOH-PF), as the emitter.[18] Compared with WOLEDscontaining multiple emitting components, those featuringa single emitter have advantages in terms of greater stabilityand reproducibility and require much simpler fabrication pro-cesses. Recently, another single-emitting-component polymerdevice that exhibited white electroluminescence (EL) was ob-tained with red emission, arising from its aggregates in con-junction with blue molecular fluorescence.[19]Although manypotentially suitable polymeric materials have been devel-oped,[20, 21] color instability (owing to phase separation of thevarious emissive components), color variation upon changingthe driving voltage, and undesired Fçrster-type energy transferbetween chromophores remain key issues that must be re-solved.[22, 23]

Herein, we report the synthesis, morphology, photolumines-cence (PL) and EL properties of 2,2’,7,7’-tetrakis(pyren-1-yl)-9,9’-spirobifluorene (TPSBF). We examined variations in the filmthickness and morphology by using AFM, and identified threeprimary modes of emission. We investigated the properties ofTPSBF through a combination of spectral analysis and the fab-rication of OLED devices.

Several chemical approaches have been reported for the de-velopment of EL materials with high glass transition tempera-tures (Tg).The syntheses of many low-molecular-weight com-pounds, including spiro-shaped, dendritic, tetrahedral, andcardo systems,[24, 25] have been pursued to confer higher stabili-

We describe how the morphology, photoluminescence (PL),and electroluminescence properties of 2,2’,7,7’-tetrakis(pyren-1-yl)-9,9’-spirobifluorene (TPSBF) are related to its film thicknessand how, under optimized conditions, the three main PL emis-sions constitute a form of white light. We fabricated high-brightness, broad-spectrum, white-light organic light-emittingdiodes, incorporating a single emission layer of the blue-light-emitting material TPSBF, which comprises four pyrene unitslinked to a spirobifluorene core. An organic light-emitting

device with the configuration indium tin oxide (170 nm)/4,4’,4’’-tris[N-(2-naphthyl)-N-phenylamino]triphenylamine (15 nm)/4,4’-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (65 nm)/TPSBF(50 nm)/tris(8-hydroxyquinoline)aluminum (30 nm)/LiF (0.8 nm)/Al(200 nm) exhibited a broad-spectrum white emission with a maxi-mum luminescence and current efficiency of 57 680 cd m�2 and6.51 cd A�1, respectively, and Commission International Del’Eclairage coordinates of (0.29,0.36).

[a] H.-Y. Wen, C.-M. Chao, Prof. M.-Y. ChangDepartment of PhotonicsNational Sun Yat-Sen UniversityKaohsiung 804 (Taiwan)E-mail : mychang2009@yahoo.com.tw

[b] C.-W. Hsieh, Prof. S.-C. HsiehDepartment of ChemistryNational Sun Yat-Sen UniversityKaohsiung 804 (Taiwan)

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cplu.201300088.

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ty to the glass state.[26] The molecular design reported hereincombines the high PL efficiency of pyrene moieties with thedecreased crystallinity that results from their covalent linkageto a spirobifluorene core. Electron-rich pyrene derivatives havebeen used recently to enhance the electron-transporting prop-erties of OLEDs.[27, 28] The large conjugated aromatic surface ofpyrene not only improves the electron-injection ability relativeto that of oligo- and polyfluorene derivatives,[29] but also pro-vides high PL efficiency[30] and high carrier mobility. Neverthe-less, pyrene itself is not suitable for use as a blue emitter inOLEDs because it readily crystallizes. Spiro linkages can beused to modify the steric demand of low-molecular-weight or-ganic compounds and, thereby, improve their processabilityand morphology. Because their electronic properties are re-tained, they can act as excellent single-emitting componentsfor WOLEDs. In this study, we prepared a device that incorpo-rated the spiro compound TPSBF; it exhibited a highly efficientwhite-light emission with an extremely low turn-on voltage(3 V) and a maximum brightness of 57 680 cd m�2 at 12 V. Thisdevice provided a maximum luminescence efficiency of6.51 cd A�1 (4.07 lm W�1) with Commission International Del’Eclairage (CIE) coordinates (x,y) of (0.29,0.36) (see Figure S1 inthe Supporting Information for the TPSBF synthetic procedureand electroluminescence spectra of device II at different lumi-nescence values).

Results and Discussion

Absorption and emission spectroscopy

We prepared TPSBF (see Scheme 2 below) readily and in highyield by using a traditional Suzuki reaction, and confirmed itschemical structure through 1H and13C NMR spectroscopy andmass spectrometry. Figure 1 displays the absorption and PLspectra of TPSBF as a dilute solution in CHCl3 and asa vacuum-evaporated thin film. Upon UV irradiation at l=

358 nm in CHCl3 (10�4m), the onset of absorption appeared at

l= 415 nm. TPSBF exhibits intense blue fluorescence, with anemission peak centered at l= 421 nm in solution. These resultshave been published in previous studies.[31] The fluorescencespectrum of a dilute solution of TPSBF, typical of the isolatedmolecule, reveals a fine structure that results from vibroniccoupling.[32] In the solid state (thin film prepared through evap-oration), the blue emission of TPSBF was redshifted to l=

456 nm and the peak broadened, reflecting local interactionsbetween neighboring chromophores; an additional broadband emission appeared at l= 500–550 nm, tailing to l=

620 nm. Redshifted emissions are typically observed in the PLspectra of solid organic and polymeric materials as a result ofintermolecular interactions or exciton hopping in the solidstate;[33] the appearance of an orange emission, however, is in-teresting and warrants further investigation. The appearance ofan additional band gap in the solid can result from aggrega-tion, phosphorescence, or excimer formation. To probe the ori-gins of the long-wavelength broad emissions near l= 500–550 nm, we studied the PL excitation, absorption, and solid-state thin-film spectra of TPSBF.

In the solid state, we observed a vibronic emission patternsimilar to that in solution, with the maxima redshifted by a fewnanometers. The PL signal of the film at l= 456 nm was similarto that found in solution, except that the former was redshift-ed by approximately 35 nm. We attribute this redshift to a var-iation in the dielectric constant of the environment, which isoften observed for organic materials.[34] Thus, we ascribe theemission band at l= 456 nm to isolated TPSBF molecules. Fewdifferences existed between the absorption spectra of the solu-tion and the film. The absorption signal in the film was red-shifted by 10 nm from that of the dilute solution in CHCl3, im-plying that interactions exist in the solid state between TPSBFmolecules in the ground state and suggesting the possible for-mation of ground-state aggregates. The broad emission bandin Figure 1 stems from species that appear to exist only in theexcited state (i.e. , no absorption occurred in this spectralregion). Therefore, we can treat any aggregate state formedbetween subunits of the solid as the source of the additionalluminescence band.[35] To determine whether excimers result-ing from molecular aggregates were responsible for the long-wavelength emissions at l= 500–620 nm, we measured the PLexcitation spectra of a TPSBF film after irradiation at l= 358,456, and 500 nm (Figure 2). The PL emissions in the three exci-tation spectra were slightly different, with the excitation-select-ed band shifting toward longer wavelengths upon increasingthe excitation wavelength, indicating that the differences inthe three excitation spectra resulted from somewhat differentexcitation pathways formed from conjugated segments thatwere in close contact with one another by extension of theirp systems.

These spectral observations suggested that our new fluoro-phore might form aggregates in the film state.[36] We also stud-ied the film-forming properties of TPSBF. Figure 3 displays thefluorescence spectra of TPSBF thin films with thicknesses ofl= 30, 50, and 70 nm after irradiation at l= 358 nm. Threeprincipal emission bands appeared near l= 456, 500, and550 nm; we ascribe these signals to excitonic emission from

Figure 1. Absorption and PL spectra of TPSBF as a dilute solution in CHCl3

and as a vacuum-evaporated thin film.

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the mono-chromophore and to two excimers that result frommolecular aggregates, respectively. In contrast to the absorp-tion spectra, which changed only slightly upon varying thefilm thickness, the long-wavelength band and the relative in-tensities in the emission spectra varied dramatically. The rela-tive intensity of the longer-wavelength emission band in-creased slightly upon increasing the thickness of the TPSBFfilm.

The bands at l= 500 and 550 nm, which were much broaderthan those at l= 456 nm, resulted from aromatic ring interac-tions (excimers) in the solid. The excimers result when excita-tion energies delocalized over two or more neighboring chro-mophores on different molecules are arranged so that thewave functions of their aromatic rings overlap.[37, 38]

The evidence from the spectra recorded at different thick-nesses suggested that the bands consisted of at least threeoverlapping peaks. Mathematical decomposition, assumingGaussian shapes for all of the component peaks, revealed reg-

ularity in the peak spectral positions. All of the spectra weredescribed reasonably by three excimer peaks located at l =

456, 500, and 550 nm (Figure 4).[39] The observed strengths ofthese emissions, both in absolute magnitude and relative to

Figure 2. Excitation spectra of TPSBF films monitored at l = 362, 457, and500 nm.

Figure 3. Fluorescence spectra of 30, 50, and 70 nm thick TPSBF films.

Figure 4. Mathematical decomposition of the fluorescence spectra of thea) 30, b) 50, and c) 70 nm thick TPSBF films.

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the exciton band, were variable and depended on the deposi-tion conditions and thickness of the sample. The relative inten-sity of the signal at longer wavelength grew upon increasingthe thickness, with a redshift of the emission maximum. Ac-cordingly, we suspected that the redshift was probably due tothe molecule of TPSBF having a longer conjugated length, re-sulting in an increasing ratio for the exciton to excimer (aggre-gate) emissions.

Morphological analysis

We suspected that the degrees of aggregation of the TPSBFmolecules affected the morphologies of the films of variousthicknesses. Therefore, we used AFM to characterize the sur-face morphologies of these films. We grew thin films withthicknesses of 0.5, 1, 5, 30, 50, and 70 nm on silicon substratesthrough vacuum deposition; Figure 5 displays the correspond-ing AFM images. Significant morphological changes occurredupon varying the film thickness. In Figure 5 a, the surface ofthe 0.5 nm film exhibited an apparently granular phase, whichpresumably represented several TPSBF molecules on the sub-strate. The 1.0 nm thin film presented features with more parti-cles forming elliptic shapes (Figure 5 b). The AFM images re-corded in phase mode revealed that the interactions betweenthe molecules were stronger than those between the moleculeand substrate. Therefore, the presence of the pyrene moietieson the difluorene backbone enhanced the interactions be-tween the molecules and, thereby, their physical aggregation,leading to redshifted fluorescence and relaxation through exci-mers.[40] To verify these assumptions, we compared the surfacefeatures of the 30 and 70 nm thick films. The surface of the70 nm thick film was quite rough with many small crystallitesas well as some stripes (Figure 5 f).

Indeed, the film-forming properties of the 70 nm thick filmwere much poorer than those of the 30 nm thickfilm. The degree of aggregation increased slightlyupon increasing the thickness from 30 to 70 nm (Fig-ure 5 d–f). Figure 6 reveals the three-dimensional sur-face features of the 30, 50, and 70 nm thick films; thedegree of aggregation increased slightly upon in-creasing the thickness. We suspected that the degreeof physical aggregation increased as a result of p–p

interactions between the pyrene moieties and led tobroadband emission (represented schematically inFigure 9 below).[27, 40]Accordingly, we attribute theemissions at l= 500 and 550 nm in the fluorescencespectra (Figure 4) to the formation of aggregates.[41]

Therefore, the degree of aggregation in the filmsinfluenced the emission spectra. As a result, we could vary thefilm thickness of this molecule to optimize the performance ofsingle-layer light-emitting white OLED.

Electrical characteristics of devices

We determined the electrochemical properties of TPSBF byusing a Riken KeiKi AC-2 photoelectron spectrometer. TheHOMO energy level of TPSBF was 5.7 eV; the LUMO energy

level, calculated from the HOMO energy level and the energygap (Eg) from the edge of the absorption spectrum, was ap-proximately 2.6 eV. To evaluate the applicability of using TPSBFas the emitting layer in WOLEDs, we recorded the EL spectraof the thin TPSBF films to determine the degrees of aggrega-tion in the emissive layers.

To investigate the EL properties of TPSBF, we fabricatedthree nondoped devices (I–III, Figure 7) with the structure ITO(170 nm)/2T-NATA (15 nm)/NPB (65 nm)/TPSBF (x nm)/Alq3

(30 nm)/LiF (0.8 nm)/Al (200 nm). In these devices, ITO and LiF/Al were the anode and the cathode, respectively, and 2T-NATA,NPB, and Alq3 were used as the HIL, HTL, and ETL, respectively.We used TPSBF as the material for the emitting material layer(EML), varying its thickness from 30 to 70 nm. As revealed inFigure 8, the overall EL patterns in the spectra of devices I–IIIchanged slightly when we increased the thickness of theTPSBF film from 30 to 70 nm, with the intensities of the emis-sions at l= 500 and 600 nm increasing accordingly. As was thecase in the PL spectra, the slight redshift upon increasing thethickness from 30 to70 nm suggested that the pyrene unitswere involved in stronger interactions, resulting in redshiftedfluorescence and enhanced physical aggregation of the TPSBFmolecules.

The long-wavelength emissions near l= 500–600 nm in theEL spectra (Figure 8) were stronger than those in the PL spec-tra of the TPSBF films (Figure 3); we suspected that the recom-bination region was closer to the cathode when the thicknessof the emitting layer increased. When we increased the thick-ness from 30 to 70 nm, the PL emission at l= 500–550 nmbecame more intense (cf. Figure 3); the corresponding AFMimages revealed increased degrees of aggregation. We sus-pected that the increased PL emission at l= 500–550 nm arosebecause of greater degrees of aggregation. Table 1 shows themeasurement results of devices I, II, and III. Therefore, the dif-

ferences in emission intensities in the EL and PL spectra mayhave been due to variations in the nature of the recombina-tion region. In device II, which featured the nondoped single-emitting layer, the emission was close to white, with CIE coor-dinates of (0.29,0.36). The current efficiency–current densitycharacteristics of device II (Figure 9 a) reveal a maximum effi-ciency of 6.51 cd A�1 at 5.4 mA cm�2 ; Figure 9 b reveals a powerefficiency of 4.07 lm W�1 at 4.5 V, with the current density–volt-age characteristics presented in the inset. The brightness of

Table 1. Luminance characteristics of the white OLEDs based on a single emittinglayer of devices I, II, and III.[a]

Device hext

[%, V]hc*[cd A�1]

hL*[cd m�2]

hp*[lm W�1]

hc[cd A�1]

hL[cd m�2]

hp[lm W�1]

CIE[b]

(x,y)

I 2.24, 5.5 5.69 45 530 3.88 5.59 1310 2.93 (0.27,0.35)II 2.52, 6.0 6.51 57 680 4.07 6.41 1297 3.10 (0.29,0.36)III 2.40, 7.5 6.41 60 230 3.28 6.32 1410 2.48 (0.32,0.41)

[a] The data for external quantum efficiency (hext), current efficiency (hc*), luminance(hL*), and power efficiency (hp*) are the maximum values of the device; current effi-ciency (hc), luminance (hL), and power efficiency (hp) were recorded at a current den-sity of 20 mA cm�2. [b] EL was measured at 7 V.

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device II was 57 680 cd m�2 when driven at 12.0 V (Figure 9 c);this value is the highest reported for a nondoped single-emit-ting layer WOLED. To the best of our knowledge, this bright-ness is also the highest ever reported for a small-molecule-based fluorescent WOLED. For comparison, a nondoped single-

emitting-layer WOLED reported in 2011 exhibited a maximumefficiency of 3.10 cd A�1, a power efficiency of 2.20 lm W�1,a maximum brightness of 1092 cd m�2, and CIE coordinates of(0.34,0.29).[42]

Figure 5. AFM images of TPSBF films with thicknesses of a) 0.5, b) 1, c) 5, d) 30, e) 50, and f) 70 nm.

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Conclusion

Without the need to add any dopant, the fluorescent smallmolecule TPSBF is capable of emitting white light from single-emitting-component EL devices, the emitting color of whichwas independent of the thickness of the TPSBF film. Thewhite-light emission resulted from three emitting wavelengthsat l= 458, 500, and 550 nm, with long-wavelength emissionsat l= 500–620 nm that resulted from molecular aggregates.We fabricated three EL devices that incorporated TPSBF as thematerial for the emitting layer at thicknesses of 30, 50, and70 nm. Device II emitted bright white light efficiently, witha maximum luminescence and a current efficiency of

57 680 cd m�2 and 6.51 cd A�1, respectively; its CIE coordinateswere (0.29,0.36).

Experimental Section

Materials

All solvents were purified by routine procedures. Other reagentswere used as received from commercial sources. The chemicalstructure and the synthetic procedure toward TPSBF are presentedin Scheme 1. Suzuki coupling of pyrene boronic acid (prepared ac-

cording to a reported process)[43] and 2,2’,7,7’-tetrabromo-9,9’-spi-robifluorene in toluene and EtOH provided TPSBF in a yield of60 % after flash column chromatography.[44] The molecular struc-ture of TPSBF was confirmed through characterization by 1H and13C NMR spectroscopy and MALDI-TOF MS.

Measurements

MALDI-TOF mass spectra were recorded by using a Bruker Proflexmass spectrometer. Thermogravimetric analysis (TGA) was per-

Figure 6. Schematic representation of the p–p stacking of pyrene groupsand three-dimensional AFM images of TPSBF films with thicknesses of 30,50, and 70 nm.

Figure 7. Configurations and energy levels of the OLED devices prepared inthis study.

Figure 8. EL spectra of devices I (30 nm of TPSBF), II (50 nm of TPSBF), and III(70 nm of TPSBF).

Scheme 1. Molecular structure and synthetic procedure for the synthesis of2,2’,7,7’-tetrakis(pyren-1-yl)-9,9’-spirobifluorene (TPSBF).

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formed by using a PerkinElmer thermogravimeter (model PYRIS1 TGA) under a dry N2 gas flow at a heating rate of 10 8C min�1. PLand absorption spectra were recorded by using a PerkinElmer FLLS55 fluorescence spectrophotometer and a PerkinElmer

Lambda35 UV/Vis spectrometer, respectively. The energy levels ofthe HOMO were determined by using a Riken KeiKi AC-2 photo-electron spectrometer; the energy level of the LUMO was deter-mined from the HOMO energy level and the lowest energy absorp-tion edge in the UV absorption spectrum. The EL spectra and CIEcoordinates of the devices were measured by using a PR650 spec-trometer. The fluorescence quantum yield was measured by usinga Hamamatsu C9920-02 absolute PL quantum yield measurementsystem. The luminance–current density–voltage characteristicswere recorded simultaneously with the EL spectra by combiningthe spectrometer with a Keithley 2400 programmable voltage–cur-rent source after the devices had been sealed in a nitrogen-filledglove box. All measurements were made at room temperatureunder ambient conditions. The active area of the device was3 mm2. The surface morphologies of thin films on silicon (100)wafer substrates (TSR Technology; P-type/boron dopant) weremeasured by using an MFP-3D atomic force microscope (AsylumResearch, Santa Barbara, CA). A silicon cantilever (Olympus, AC240)with a nominal spring constant of 1 N m�1 was used for all images,

Figure 9. a) EL efficiency–current density, b) power efficiency–current density(inset: current density–voltage), and c) EL brightness–current density charac-teristics of the TPSBF-based device II.

Scheme 2. Molecular structures of TPSBF, 2T-NATA, NPB, and Alq3.

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recorded at a scan rate of 1.0 Hz and an image pixel density of512 � 512.

OLED fabrication

Scheme 2 presents the molecular structures of TPSBF, 4,4’,4’’-tris[N-(2-naphthyl)-N-phenylamino]triphenylamine (2T-NATA), 4,4’-bis[N-(1- naphthyl)-N-phenylamino]biphenyl (NPB), and tris(8-hydroxyqui-noline)aluminum (Alq3). The devices were fabricated on glass sub-strates layered with 1700 � thick indium tin oxide (ITO) witha sheet resistance of about 10 W/&. Prior to deposition, the sub-strates were cleaned through ultrasonication in isopropanol anddeionized water, followed by treatment with O2 plasma. The ITOsubstrate was then loaded into a deposition chamber. All of the or-ganic layers and the LiF/Al layers were evaporated onto the sub-strates at 10�6 torr without breaking the vacuum. The rates ofevaporation and the thicknesses of the organic and metal layerswere monitored by using quartz oscillators. The device structurewas ITO (170 nm)/2T-NATA (15 nm)/NPB (65 nm)/TPSBF (x nm; x =

30, 50, 70)/Alq3 (30 nm)/LiF (0.8 nm)/Al (200 nm); the pertinentenergy levels are displayed in Figure 7. 2T-NATA was used as thehole injection layer (HIL); NPB and Alq3 were used as the holetransporting layer (HTL) and electron transporting layer (ETL), re-spectively. The emission layer was fabricated through evaporationof TPSBF. ITO and LiF/Al were employed as the anode and cathode,respectively.

Acknowledgements

We thank the National Science Council, Taiwan (grantno. NSC 101-2221-E-110-082) for financial support.

Keywords: luminescence · photochemistry · spirocompounds · thin films

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